Improved variable resistor



sta s Patent 2,978,663 IMPROVED VARIABLE RESISTOR No Drawing. Filed Aug.12, 1959, set. No. 833,129 Claims. or. 338-160) This invention relatesto an improvement in electrical contacts. More particularly, it relatesto a means for prolonging the satisfactory life of electrical contactsin which one member makes contact with a second member by wiping thesurface of the second member, as is common in rheostats andpotentiometers.

One object of the invention is to prolong the life of such contactsparticularly when exposed to service conditions involving elevatedtemperatures.

Another object of the invention is to preserve the integrity of thecontact materials in service, and to minimize Wear of the contactmaterials, without impairing the efficiency of the electrical connectionbetween them and without concurrently reducing the pressure of the wipermember on the surface wiped.

Still a further object is to reduce the torque which is required to movethe wiper along a conductive track member without altering the design ofthe parts, and while permitting the use of a simple wiping type ofcontact.

These and other objects will become apparent from the description of theinvention which follows.

In general the devices to which the invention is applicable include anelectrically insulating base portion formed preferably of a high meltingpoint glass or ceramic material on which there is supported a thin filmof an electrically conductive material having any suitableconfiguration. The problem of galling is particularly acute with theextremely thin films commonly used in resistors. and like devices,particularly when these devices are operated at relatively hightemperatures such as 500 C.

Hence the invention is applicable to devices in which the electricallyconductive portion may comprise a thin film, for example one depositedby evaporation of a metal, metal alloy, or other suitable compound orcomposition; or it may comprise an electrically conductive glaze whereina continuous metallic network pervades a ceramic matrix as described inan application, Serial Number 833,127 filed of even date herewith; or itmay comprise any of the various electrically conductive metal oxidecompositions.

For purposes of illustration the invention will he described withreference to thin films consisting of tin oxide, containing antimonyoxide in small amounts, but it is to be understood that such films areillustrative of but one preferred embodiment and are not to be construedas limitative.

Thus, in this instance, the electrically conductive portion is formed ofa thin film of tin oxide or a thin layer of a composition based on tinoxide, deposited as a film about 0.0001 inch or thinner on theelectrically insulating substrate by techniques already known anddescribed for example in United States Patent 2,118,975 and 2,567,331,among others.

Such coatings, and more particularly, tin oxide and tin oxide-antimonyoxide coatings as the electrically conductive track are known to becapable of carrying high cur- 2 rents and to possess many advantagesindicated in United States Patent 2,564,706 and have been found topossess, in addition, superior stability at temperatures in the vicinityof 500 C.

The second portion of such devices includes a wiper, which may beeithersingle or multiple and mounted by any suitable means so that atleast a portion of the wiper is urged firmly into physical contact withthe surface of the electrically conductive portion. With theelectrically conductive portions formed of thin films as describedabove, and particularly in service applications at elevatedtemperatures, the wiper materials usually employed, e.g., platinumalloys or gold alloys or other precious metal alloys, when pressedagainst and moved along the electrically conductive portion have beenfound to gall the film material and to seize thereon, and the actualdetachment of pieces of the extremely thin film of the electricallyconductive material from the base has been experienced. As a result,when using sufiicient pressure to obtain a good electrical contact, theWiper may often destroy the coating and consequently the resistor unititself, by such seizing and galling.

One previously devised approach to the solution of the problem has beento diminish the amount of pressure bringing the wiper arm into physicalcontact with the base. This has not always been satisfactory because itoften produces noise as the contact moves past microscopicirregularities and unevennesses in the surface of the material.

Other previously proposed solutions to the problem have involved aredesign of the track or the wiper, but this is objectionable for, oneor more of the following reasons: it may add to the cost of manufacture;increase the noise developed by the device in use; reduce thereliability of the contact system; impair resolution; and magnifyeffects of dirt and wear particles.

By the present invention the problems of sticking, seizing and gallingare overcome, either by the addition of suitable materials to theelectrically conductive film or by the addition of the same materials tothe wiper. The material may be added by rubbing or burnishing it ontothe already formed members, or'it may be otherwise incorporated into themembers when they are being formed, for example by sintering a shapedmixture comprising particles of a precious metal alloy admixed with leadoxide or other suitable anti-galling material.

Other materials found to possess suitable physical, chemical andelectrical properties for the present purposes include the following:graphite; boron nitride; litharge (lead oxide); and molybdenumdisulfide. The molybdenum disulfide and graphite are effective for thepresent invention for somewhat lower temperatures up to about 275 C.,but the remaining materials may be used to SOD-600 C.

As indicated above, a preferred method of application of these and othermaterials possessing similarly suitable properties is by rubbing orburnishing fine particles of these materials onto the electricallyconductive film of the otherwise completed device against which a wiperrubs in the normal service of the device. Preferably the fine particlesare impalpable powder, the particles of which have no dimension largerthan about 5 microns.

No vehicle or dispersing agent is required to facilitate a uniformdistribution, but may be used if the method of application chosenincludes spraying the particles onto the resistor surface.

The amounts applied are insuflicient to impair the electrical contactbetween the wiper and the electrically conductive material, but stillmust be suflicient to diminish any tendency to seize or gall.

Resistors embodying the present invention have been formed as follows: iI

A coating composition has been prepared by mixing the following in abeaker:

25 g. SHCl4'5H2O 0.75 g. SbCl 10 Cc. H20

5 cc. HCl (concentrated) An electrically conductive track has been laiddown on an electrically insulating ceramic base material by heating thebase to between 600 C. and 1200 C., depending on the specific basematerial used, and spraying a solution of the above composition onto theheated base.

With a thin ceramic wafer substrate, to obtain an even distribution ofthe coating, it was found desirable to mount the wafer on a rotatablesupport in an electrically heated kiln and to spray the coating mist andonto the water while it is in the furnace and after it had been held attemperature for between 5 and minutes.

The wafer and adherent irridescent coating were permitted to cool andthereafter, finely divided (minus 5 microns) litharge (PbO) was rubbedinto the tin oxide coating using a piece of felt as the applicator. Inalternative embodiments at least about by weight and preferably between20% and about 50% by weight of lead oxide was admixed with a preciousmetal alloy, based on either platinum or gold, then shaped to form acontact and sintered to obtain the wiper.

With a resistor consisting of a glass substrate and a tin oxide-antimonyoxide coating thereon formed as described above, and a wiper composed of90% Pt-l0% Rh operated with the wiper spring pressed on the track with aforce of -60 grams, and with the wiper oscillated at a uniform speedbetween spaced terminals at either end of the conductive track, thefollowing comparative results were obtained:

If any tendency to gall or seize is noted, additional powder may beapplied to the electrically conductive film track, from time to time bymerely repeating the application described above.

Having now described the invention with the particularity required bythe patent statutes, we claim:

1. In a variable resistor comprising an electrically insulatingsubstrate, a composition consisting principally of tin oxide supportedon said substrate as an electrically conductive coating and a metalwiper adapted to move along and rub against said composition, theimprovement which comprises diminishing the seizing and galling betweensaid wiper and said coating by providing a material of the groupconsisting of lead oxide (PbO), boron nitride, molybdenum disulfide andgraphite at the interface between said wiper and said coating.

2. The device of claim 1 wherein the material is provided as aconstituent of a sintered metal wiper.

3. The device of claim 1 wherein the material is provided on the surfaceof the electrically conductive coating.

4. In a variable resistor comprising an electrically insulatingsubstrate, a composition consisting principally of tin oxide supportedon said substrate as an electrically conductive coating and a metalwiper adapted to move along and rub against said composition, theimprovement which comprises diminishing the seizing and galling betweensaid wiper and said coating by providing lead oxide (PhD) at theinterface between said wiper and said coating.

5. In a variable resistor comprising an electrically insulatingsubstrate, supporting and integral with a thin film of electricallyconductive coating and a metal wiper adapted to move along and rubagainst said coating, the improvement which comprises diminishing theseizing and galling between said wiper and said coating by providing amaterial of the group consisting of lead oxide (PbO), boron nitride,molybdenum disulfide and graphite at the interface between said wiperand said coating.

References Cited in the file of this patent UNITED STATES PATENTS2,620,555 Lenz Dec. 9, 1952 2,632,831 Pritikin et a1. Mar. 24, 19532,699,839 Hamel Ian. 18, 1955 2,702,092 Douglass Feb. 15, 1955 2,750,527Katz June 12, 1956 2,760,036 Raymer Aug. 21, 1956 UNITED STATES PATENTOFFICE CERTIFIQATE (H @QRRECTWN Patent No, 2 978 663 April 4; 1961Arnold 55., Lewis et :11,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

In the heading to the printed speeifieetiom line 4 address of inventorsfor e/e Market Company read mm c/o Markite Company Signed and sealedthis 5th day of September 19610 (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L LADD Commissioner of PatentsUSCOMM-DC UNITE STATES PATENT FHCE @ETEMQATE @F CQREQHGN Patent No, e emeee April 1%1 Arnold So Louis ei; e10

It is hereby certified that error appears in the above numbered pat entrequiring correction and that "the said Letters Patent should read ascorrected below,

In the heading to the printed specificetion line 4 address of inventorsfor 0/0; Market Company read 0/0 Markite Company e Signed and sealedthis 5th day of September 19610 (SEAL) Attest:

ERNEST W. SDER DAVID L. L :l:

Attesting Ufficer Commissioner of Patents USCOMM-DC

1. IN A VARIABLE RESISTOR COMPRISING AN ELECTRICALLY INSULATINGSUBSTRATE, A COMPOSITION CONSISTING PRINCIPALLY OF TIN OXIDE SUPPORTEDON SAID SUBSTRATE AS AN ELECTRICALLY CONDUCTIVE COATING AND A METALWIPER ADAPTED TO MOVE ALONG AND RUB AGAINST SAID COMPOSITION, THEIMPROVEMENT WHICH COMPRISES DIMINISHING THE SEIZING AND GALLING BETWEENSAID WIPER AND SAID COATING BY PROVIDING A MATERIAL OF THE GROUPCONSISTING OF LEAD OXIDE (PBO), BORON NITRIDE, MOLYBDENUM DISULFIDE ANDGRAPHITE AT THE INTERFACE BETWEEN SAID WIPER AND SAID COATING.